Radio communication apparatus, radio communication method, and non-transitory computer readable medium storing radio communication control program

ABSTRACT

A combination of antennas to be used is selected based on a distance between a plurality of antennas or the polarization direction of a radio signal to be transmitted/received.

INCORPORATION BY REFERENCE

This application is based on Japanese Patent Application No. 2009-253257filed on Nov. 4, 2009 and including specification, claims, drawings andsummary. The disclosure of the above Japanese Patent Application isincorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a radio communication apparatus and aradio communication method, which can change the antenna configurationat the time of carrying out spatial-multiplexing based communication,and a recording medium recording a radio communication control programwhich can change the antenna configuration at the time of carrying outspatial-multiplexing based communication.

BACKGROUND ART

In the 3GPP (3rd Generation Partnership Project) which is theinternational standardization body for the third generation mobilecommunication systems, standardization of E-UTRA (Evolved-UMTSTransmitter Radio Access) is fostered. The E-UTRA aims at speeding upthe 3.5 generation, UTRA, and is positioned as the 3.9 generation calledLTE (Long Term Evolution).

In the LTE, communication is carried out based on spatial multiplexingsuch as MIMO (Multi-Input Multi-Output) to achieve fast and massinformation transfer and improve the frequency use efficiency. Further,the fourth generation, IMT-Advanced, which involves a greater number ofspatial multiplexing channels and a wider band has been studied. TheMIMO is the communication system that uses a plurality of antennas toincrease paths based on spatial multiplexing and improve the throughput.This communication system also uses the same frequency to provide asatisfactory frequency use efficiency. MIMO whose number of inputs is aand whose number of outputs is b is called a×bMIMO. In the LTE, forexample, the quantity of antennas at a base station (maximum spatialmultiplexing channels) and the quantity of antennas of a terminal(maximum of four), which is 2×2MIMO (4×4MIMO at a maximum).

A terminal in the LTE acquires the quantity of antennas at a basestation from information included in a PBCH (Physical Broadcast CHannel)which is a common control channel to globally inform control informationunique to systems and cells. This terminal uses an RS (Reference Signal)to calculate a spatial matrix from the RS received at each antenna, usesa PUCCH (Physical Uplink Control CHannel) to inform the base station ofa PMI (Precoding Matrix Indicator), RI (Rank Indicator), and CQI(Channel Quality Indicator). The base station decides the precoding andtransmission mode based on the PMI, RI and CQI, and uses a PDCCH(Physical Downlink Control CHannel) to inform the terminal of the resultof the decision.

When there are a large number of low-correlation spatial multiplexingchannels available, for example, communication at a maximum of about 300Mbps is possible in DL (Down Link). When the radio wave environment ispoor, however, a base station carries out communication withtransmission diversity in transmission mode. In the latter case,transmission of same data from two antenna ports may not improve thethroughput, but can enhance the redundancy to achieve stablecommunication.

Unexamined Japanese Patent Application KOKAI Publication No. 2008-166855discloses the configuration that carries out communication using aplurality of antennas.

With the speed of mobile communication improved, the use of spatialmultiplexing typified by MIMO further improves the throughput and thefrequency use efficiency. However, increasing the number of spatialmultiplexing channels brings about various problems such that thequantity of antennas is increased, and dissipation power is increased byan increase in the amount of signal processing. Therefore, there is alimit to the quantity of antennas mountable to a portable terminal whichis used in mobile communication.

It is therefore desirable to increase the quantity of antennas incarrying out communication. However, the technique described inUnexamined Japanese Patent Application KOKAI Publication No. 2008-166855is about where antennas are mounted, such as an external antennaslementis provided at the battery pack of a cellular phone. It is thereforedifficult to keep a satisfactory communication state by flexiblychanging the antenna configuration according to various situations.

SUMMARY

Accordingly, it is an exemplary object of the present invention toprovide a satisfactory communication state by flexibly changing theantenna configuration to be used in spatial-multiplexing basedcommunication according to various situations.

To achieve the object, according to a first exemplary aspect of theinvention, there is provided a radio communication apparatus includingan antenna set including a plurality of antennas and executingspatial-multiplexing based communication, including:

an antenna selection unit that selects from the antennas a combinationof antennas whose quantity is equal to or less than a quantity ofcounterpart antennas to be communicated, as a combination of antennas tobe used in spatial-multiplexing based communication;

a signal processing unit that executes signal processing forcommunication using the combination of antennas selected by the antennaselection unit; and

an antenna configuration specifying unit that specifies a physicalconfiguration of the plurality of antennas,

the antenna selection unit selecting the combination of antennas to beused based on the configuration selected by the antenna configurationspecifying unit.

The antenna configuration specifying unit may specify a physicalinter-antenna distance as the physical configuration of the plurality ofantennas, and

the antenna selection unit may select a combination of antennas whichmaximizes the physical inter-antenna distance specified by the antennaconfiguration specifying unit.

The antenna configuration specifying unit may specify a polarizationdirection of a radio signal to be transmitted/received by each antenna,and

the antenna selection unit may select a combination of antennas withdifferent polarization directions specified by the antenna configurationspecifying unit.

To achieve the object, according to a second exemplary aspect of theinvention, there is provided a radio communication apparatus includingan antenna set including a plurality of antennas and executingspatial-multiplexing based communication, including:

an antenna selection unit that selects from the antennas a combinationof antennas whose quantity is equal to or less than a quantity ofcounterpart antennas to be communicated, as a combination of antennas tobe used in spatial-multiplexing based communication;

a signal processing unit that executes signal processing forcommunication using the combination of antennas selected by the antennaselection unit; and

a reception quality detecting unit that detects a reception quality foreach combination of a plurality of antennas,

the antenna selection unit selecting the combination of antennas to beused according to a result of comparison of reception qualities detectedby the reception quality detecting unit.

The radio communication apparatus may further include a communicationenvironment detecting unit that detects a predetermined amount ofvariation in communication environment of the radio communicationapparatus,

wherein the antenna selection unit may vary a period of changing acombination of antennas to be selected, depending on whether the amountof variation detected by the communication environment detecting unit isequal to or less than a predetermined reference value.

The radio communication apparatus may further include a communicationenvironment change detecting unit that detects a time-dependent changein a predetermined amount of variation in communication environment ofthe radio communication apparatus,

wherein the antenna selection unit may vary a period of changing acombination of antennas to be selected, depending on whether thetime-dependent change in the amount of variation detected by thecommunication environment change detecting unit is equal to or less thana predetermined reference value.

The signal processing unit may include a plurality of signal processingcircuits that process signals to be transmitted/received in associationwith the a plurality of antennas included in the antenna set, and

power supply to that one of the plurality of signal processing circuitswhich is not used may be stopped based on the combination of antennasselected by the antenna selection unit.

The signal processing unit may include a plurality of signal processingcircuits that process signals to be transmitted/received in associationwith the a plurality of antennas included in the antenna set, and

a setting value for carrying out communication with the antennas to becommunicated or another antenna to be communicated may be acquired byusing an antenna which is not selected by the antenna selection unit andthat signal processing circuit which is associated with the antenna.

The Antenna Set May Include:

a first antenna set including a plurality of first antennas in whichpolarization directions of adjacent antennas are orthogonal to eachother; and

a second antenna set including a plurality of second antennas in whichpolarization directions of adjacent antennas are orthogonal to eachother.

The Antenna Set May Include:

a first antenna set including a plurality of first antennas which causepolarization directions of signals to be transmitted/received areidentical to each other; and

a second antenna set including a plurality of second antennas whichcause polarization directions of signals to be transmitted/received areidentical to each other, and

the polarization direction of the signals to be transmitted/received bythe plurality of first antennas may be orthogonal to the polarizationdirection of the signals to be transmitted/received by the plurality ofsecond antennas.

The first antenna set or the second antenna set may be configured to bedismountable from the radio communication apparatus, and

the radio communication apparatus may further include a mount detectionunit that detects if the first antenna set or the second antenna set ismounted to the radio communication apparatus.

The antenna selection unit may select a combination of antennas in sucha way as to always transmit/receive a signal having a certainpolarization direction.

To achieve the object, according to a third exemplary aspect of theinvention, there is provided a radio communication apparatus includingan antenna set including a plurality of antennas and executingspatial-multiplexing based communication, including:

antenna selection means that selects from the antennas a combination ofantennas whose quantity is equal to or less than a quantity ofcounterpart antennas to be communicated, as a combination of antennas tobe used in spatial-multiplexing based communication;

signal processing means that executes signal processing forcommunication using the combination of antennas selected by the antennaselection means; and

antenna configuration specifying means that specifies a physicalconfiguration of the plurality of antennas,

the antenna selection means selecting the combination of antennas to beused based on the configuration selected by the antenna configurationspecifying means.

To achieve the object, according to a fourth exemplary aspect of theinvention, there is provided a radio communication apparatus includingan antenna set including a plurality of antennas and executingspatial-multiplexing based communication, including:

antenna selection means that selects from the antennas a combination ofantennas whose quantity is equal to or less than a quantity ofcounterpart antennas to be communicated, as a combination of antennas tobe used in spatial-multiplexing based communication;

signal processing means that executes signal processing forcommunication using the combination of antennas selected by the antennaselection means; and

reception quality detecting means that detects a reception quality foreach combination of a plurality of antennas,

the antenna selection means selecting the combination of antennas to beused according to a result of comparison of reception qualities detectedby the reception quality detecting means.

To achieve the object, according to a fifth exemplary aspect of theinvention, there is provided a radio communication method of executingspatial-multiplexing based communication using an antenna set includinga plurality of antennas, including:

an antenna selection step of selecting from the antennas a combinationof antennas whose quantity is equal to or less than a quantity ofcounterpart antennas to be communicated, as a combination of antennas tobe used in spatial-multiplexing based communication;

a signal processing step of executing signal processing forcommunication using the combination of antennas selected by the antennaselection unit; and

an antenna configuration specifying step of specifying a physicalconfiguration of the plurality of antennas,

the antenna selection step selecting the combination of antennas to beused based on the configuration selected in the antenna configurationspecifying step.

To achieve the object, according to a sixth exemplary aspect of theinvention, there is provided a radio communication method of executingspatial-multiplexing based communication using an antenna set includinga plurality of antennas, including:

an antenna selection step of selecting from the antennas a combinationof antennas whose quantity is equal to or less than a quantity ofcounterpart antennas to be communicated, as a combination of antennas tobe used in spatial-multiplexing based communication;

a signal processing step of executing signal processing forcommunication using the combination of antennas selected by the antennaselection unit; and

a reception quality detecting step of detecting a reception quality foreach combination of a plurality of antennas,

the antenna selection step selecting the combination of antennas to beused according to a result of comparison of reception qualities detectedin the reception quality detecting step.

To achieve the object, according to a seventh exemplary aspect of theinvention, there is provided a recording medium recording a programallowing a computer that controls a radio communication apparatus whichexecutes spatial-multiplexing based communication using an antenna setincluding a plurality of antennas, to function as:

an antenna configuration specifying unit that specifies a physicalconfiguration of the plurality of antennas;

an antenna selection unit that selects from the antennas a combinationof antennas whose quantity is equal to or less than a quantity ofcounterpart antennas to be communicated, as a combination of antennas tobe used in spatial-multiplexing based communication, based on theconfiguration specified by the antenna configuration specifying unit;and

a signal processing unit that executes signal processing forcommunication using the combination of antennas selected by the antennaselection unit.

To achieve the object, according to a eighth exemplary aspect of theinvention, there is provided a recording medium recording a programallowing a computer that controls a radio communication apparatus whichexecutes spatial-multiplexing based communication using an antenna setincluding a plurality of antennas, to function as:

a reception quality detecting unit that detects a reception quality foreach combination of a plurality of antennas;

an antenna selection unit that selects from the antennas a combinationof antennas whose quantity is equal to or less than a quantity ofcounterpart antennas to be communicated, as a combination of antennas tobe used in spatial-multiplexing based communication, according to aresult of comparison of reception qualities detected by the receptionquality detecting unit; and

a signal processing unit that executes signal processing forcommunication using the combination of antennas selected by the antennaselection unit.

The present invention can provide a satisfactory spatial-multiplexingbased communication by changing the antenna configuration flexibly.

BRIEF DESCRIPTION OF THE DRAWINGS

The object and other objects and advantages of the present inventionwill become more apparent upon reading of the following detaileddescription and the accompanying drawings in which:

FIG. 1 is a diagram showing an example of the configuration of aterminal according to an exemplary embodiment of the invention;

FIG. 2 is a diagram showing an example of the configuration of aterminal to which an external antenna is connected;

FIGS. 3A and 3B are diagrams each showing an example of theconfiguration of an antenna switch;

FIG. 4 is a diagram showing an example of the configuration of a controlunit;

FIGS. 5A and 5B are diagrams each showing the quantity of antennas at abase station;

FIG. 6 is a flowchart illustrating an example of a process of changingthe antenna configuration according to the amount of communication dataand the remaining amount of a battery;

FIGS. 7A and 7B are diagrams each showing a configuration example ofantennas whose polarization directions are orthogonal to one another;

FIGS. 8A to 8C are diagrams each showing a configuration example ofantennas with different polarization directions;

FIG. 9 is a flowchart illustrating an example of a process of selectingantennas based on a predetermined measurement result;

FIG. 10 is a flowchart illustrating an example of a process of selectingantennas based on a reception quality report;

FIGS. 11A and 11B are diagrams each showing an example of theconfiguration of a terminal which is equipped with a sensor;

FIG. 12 is a diagram showing an example of the configuration of a cradlewith built-in antennas.

EXEMPLARY EMBODIMENT

An exemplary embodiment of the present invention will now be describedwith reference to the accompanying drawings. FIG. 1 shows oneconfiguration example of a portable terminal device using the MIMOsystem in the E-UTRA as an application to a radio communicationapparatus.

A terminal 0 shown in FIG. 1 is a radio communication apparatus as aportable terminal device, and includes an antenna selection unit 2, anRF switch 4, a signal processing unit 5, a control unit 6, a connectorunit 7, an antenna 11, an antenna 12, an RF unit 31, and an RF unit 32.In the configuration example shown in FIG. 1, the antenna 11 and antenna12 are incorporated in the terminal 0, and are mounted in such a waythat each antenna can receive radio signals.

The antenna selection unit 2 is a switch which connects or disconnectsthe antenna 11, the antenna 12 and the connector unit 7, which areincorporated in the terminal 0, to and from the RF unit 31 and the RFunit 32. Each of the RF units 31 and 32 has a capability offrequency-converting high-frequency signals each of two lines (maximumof four lines) received at the respective antennas including the antenna11 and the antenna 12. Each of the RF units 31 and 32 has two inputs xand y. Hereinafter, the input x of the RF unit 31 is also denoted as “RFunit 31 x”, and the input y of the RF unit 31 as “RF unit 31 y”.Likewise, the input x of the RF unit 32 is also denoted as “RF unit 32x”, and the input y of the RF unit 32 as “RF unit 32 y” hereinafter.

Each of the RF units 31 and 32 may include an amplifier, such as LNA(Low Noise Amplifier), so as to be able to amplify a received signal. Inaddition, each of the RF units 31 and 32 may include a weighableattenuator so as to be able to adjust the amplitude of a receivedsignal. Further, each of the RF units 31 and 32 may include a phaseshifter using, for example, a delay circuit, so that the phase of areceived signal can be adjusted. As apparent from the above, the RFunits 31 and 32 should be able to perform various kinds of signalprocessing on received signals as a pre-stage of demodulation or thelike of the signal processing unit 5. It is to be noted that the RFunits 31 and 32 may be configured to be able to perform various kinds ofsignal processing on transmission signals as a post-stage of modulationor the like of the signal processing unit 5.

The RF switch 4 can connect an input 52 of the signal processing unit 5to the ground. The signal processing unit 5 processes output signals ofthe RF units 31 and 32. For example, the signal processing unit 5 shouldperform signal processing, such as orthogonal detection,analog-to-digital (A/D) conversion or fast Fourier transform, using theoutput signals of the RF units 31 and 32, so that data transmitted in aplurality of sub carriers can be demodulated and decoded. The controlunit 6 performs various kinds of control, such as setting the individualsections and ON/OFF actions. The connector unit 7 serves to connect theterminal 0 to an external device.

FIG. 2 is a diagram shows a configuration example when an externalantenna is connected to the terminal 0. An external antenna 13 and anexternal antenna 14 has a configuration (e.g., a plug) to be connectableto (detachable from) the connector unit 7, and has frequencycharacteristics similar to those of the antennas 11 and 12. The controlunit 6 should detect the connection (mounting) of the external antenna13 and the external antenna 14 by means of a change in the electricalcharacteristic of the connector unit 7, a switch or the like. Theconnection (mounting) of the external antenna 13 and the externalantenna 14 can permit signals of not only two lines but signals of up tofour lines to be transmitted and received, so that the combination ofantennas to be used in spatial-multiplexing based communication can bechanged flexibly. When the combination of the antennas 11 and 12 isselected, the space of the terminal 0 can be saved (reduced) by removingthe external antenna 13 and the external antenna 14.

FIGS. 3A and 3B show examples of the configuration of the antennaselection unit 2. In the configuration example shown in FIG. 3A, theantenna selection unit 2 always transfers (outputs) a signal received atthe antenna 11 to the input x of the RF unit 31 (RF unit 31 x). Theantenna selection unit 2 shown in FIG. 3A includes three antennaswitches 22 to 24. The antenna switch 22 can be switched to transfer(output) a signal received at any one of the antenna 12, the antenna 12,and the external antenna 14 to the input y of the RF unit 31 (RF unit 31y). Therefore, it is possible to select an antenna which has a lowcorrelation to the antenna 11 as an antenna to be used with the antenna11. The antenna switch 23 can be switched whether or not to transfer(output) a signal received at the external antenna 13 to the input x ofthe RF unit 32 (RF unit 32 x), making it possible to select whether theexternal antenna 13 is connected to the input x of the RF unit 32 or toa terminal d to be the ground. The antenna switch 24 can be switchedwhether or not to transfer (output) a signal received at the externalantenna 14 to the input y of the RF unit 32 (RF unit 32 y), making itpossible to select whether the external antenna 14 is connected to theinput y of the RF unit 32 or to the terminal d to be the ground.

In this manner, the antenna switch 23 and the antenna switch 24 canconnect the inputs of the RF unit 32 (RF unit 32 x, RF unit 32 y) to theground. This can prevent signals received at the external antenna 13 orthe external antenna 14 from being input to the circuit board or the RFunit 32, thereby inhibiting occurrence of interference signals.

An antenna selection unit 201 with a configuration exemplified in FIG.3B may be used as the antenna selection unit 2. In the configurationexample shown in FIG. 3B, the antenna selection unit 201 can freelyselect connection of each antenna to each RF unit. That is, the antennaselection unit 201 differs in configuration from the antenna selectionunit 2 shown in FIG. 3A in that, for example, the antenna 11 is notdirectly connected to the input x of the RF unit 31 (RF unit 31 x). Theantenna selection unit 201 shown in FIG. 3B includes four antennaswitches 211, 221, 231 and 241. Each antenna switch 211, 221, 231, 241can be switched to select one of the four antennas (antenna 11, antenna12, external antenna 13 and external antenna 14) and the terminal d tobe the ground, and connect the selected element to the input of thecorresponding RF unit. The configuration of the antenna selection unit201 provides various selectable combinations of antennas, so that thecombination of antennas to be used can be selected flexibly to ensuresatisfactory communication based on spatial multiplexing.

In the terminal 0, a satisfactory spatial-multiplexing basedcommunication is established by flexibly changing the combination ofantennas to be used to carry out spatial-multiplexing basedcommunication according to various situations. To achieve the purpose,the control unit 6 reads and executes an operational program stored in astorage unit (e.g., flash memory, ROM, RAM, HDD, optical disc storagedevice, or magneto-optical disk storage device) incorporated in theterminal 0 or externally connected thereto to realize functions as shownin FIG. 4. As shown in FIG. 4, the control unit 6 should function as anantenna configuration specifying unit 61, a communication statedetecting unit 62, a using-antenna selection setting unit 63, a signalprocessing setting unit 64, etc.

The antenna configuration specifying unit 61 specifies the antennaconfiguration, such as the layout of antennas. The antenna configurationis specified by setting data or the like indicative of the physicaldistance between a plurality of antennas (antenna 11, antenna 12,external antenna 13, external antenna 14, etc.) which can be used by theterminal 0, and the directions of the individual antennas (directions ofreception polarizations).

The antenna configuration specifying unit 61 may read out setting datafrom the storage unit incorporated in the terminal 0 to specify thestructures or the like of antennas. Alternatively, the structures or thelike of antennas may be specified by reading setting data externallyinput to the terminal 0. As another option, the terminal 0 may includean input unit through which the manufacturer or user of the terminal 0can input data indicative of the structures of antennas, so that theantenna configuration specifying unit 61 may read the data input throughthe input unit to specify the structures or the like of antennas.

Further, the antenna configuration specifying unit 61 should specify thestructures or the like of antennas by detecting the connection state ofthe external antenna 13 or the external antenna 14 in the connector unit7. In addition, the antenna configuration specifying unit 61 may specifythe structures or the like of antennas based on the results of receptionat other antennas.

The communication state detecting unit 62 detects various statesrelating to the communication operation of the terminal 0. For example,the communication state detecting unit 62 should detect thecommunication state based on the connection states of the externalantenna 13 and the external antenna 14 in the connector unit 7, theselected setting of antennas in the antenna selection unit 2, theresults of processing received signals in the RF units 31 and 32, andthe signal processing unit 5, etc.

The communication state detecting unit 62 may detect the charged amount(remaining amount) of the battery of the terminal 0, the amount of datato be communicated, the inclination of the terminal 0, the moving speedthereof, the moving direction thereof, the result oftransmitting/receiving a reference signal for measurement, and the like,as a state relating to the communication operation.

The using-antenna selection setting unit 63 performs setting to select acombination of antennas to be used to carry out spatial-multiplexingbased communication from a plurality of antennas including the antenna11, the antenna 12, the external antenna 13 and the external antenna 14.For example, the using-antenna selection setting unit 63 should select acombination of antennas by sending a select control signal correspondingto the combination of antennas to be used to the antenna selection unit2 based on the antenna configuration (structures or the like) specifiedby the antenna configuration specifying unit 61. The using-antennaselection setting unit 63 should select a combination of antennas bysending a select control signal based on the state of the communicationoperation of the terminal 0 which is detected by the communication statedetecting unit 62.

The signal processing setting unit 64 performs process setting ofcommunication signals in various circuits included in the terminal 0 inassociation with the combination of antennas selected by theusing-antenna selection setting unit 63. For example, the signalprocessing setting unit 64 should change over and control the operationsof various circuits or the like by sending a changeover control signalcorresponding to the combination of antennas set by the using-antennaselection setting unit 63 to the RF switch 4, the RF unit 31, the RFunit 32 and the signal processing unit 5.

The operation of the terminal 0 which has the foregoing configurationand functions will be described below. First, the process of theterminal 0 in normal standby mode will be described. Various processesto be described below may be realized, for example, as the control unit6 in the terminal 0 reads and executes the program stored in the storageunit (e.g., flash memory, ROM, RAM, HDD, optical disc storage device, ormagneto-optical disk storage device) incorporated in the terminal 0 orexternally connected thereto to control the individual configurations ofthe terminal 0 and achieve the individual functions of the control unit6.

In normal standby mode, signal received at the antenna 11 and theantenna 12 incorporated in the terminal 0 travel through the antennaselection unit 2 to be input to the RF unit 31. At this time, processingis carried out in two lines in the terminal 0. Accordingly, the RF unit31 is enabled, and the RF unit 32 is disabled by the signal processingsetting unit 64 or the like in the control unit 6. Disabling the RF unit32 which will not be used stops power supply to the RF unit 32, reducingdissipation power. Further, the control unit 6 sets open the terminal ofthe antenna selection unit 2 which is connected to the connector unit 7for external antenna by means of the using-antenna selection settingunit 63 or the like, thereby connecting the RF unit 32 x and RF unit 32y or the inputs of the RF unit 32 to the ground.

The RF switch 4 sets the output of the disabled RF unit 32 open, andconnects the input to the signal processing unit 5 to the ground. Theoutput of the RF unit 31 is input to the signal processing unit 5, whichcalculates a usable RI and PMI from the RS transmitted from the basestation, and informs the base station of the RI and PMI using the PUCCHas CQI. The base station maps the DCI with the RI and PMI included inthe PDCCH, and informs the terminal 0 subframe by subframe. The signalprocessing unit 5 performs decoding through the process informed by thebase station. 2×2MIMO based communication using two lines can be carriedout this way.

When the external antenna 13 and the external antenna 14 are connectedto the connector unit 7 as shown in FIG. 2, by way of contrast, theterminal 0 may be configured to have four antennas. In this case, theterminal 0 can perform 4×4MIMO based communication using four lines, andensure communication twice as fast as the 2×2MIMO based communication.

A description will now be given of a case where the base station hasfour antennas. FIG. 5A exemplifies a case where a base station 104 hasfour antennas 114. In this example, the maximum number of spatialmultiplexing is four. In the terminal 0, the signal processing settingunit 64 or the like in the control unit 6 performs ON control on the RFswitch 4, and enables the RF unit 32. At this time, the set values ofthe RF unit 31 and the signal processing unit 5 which have already beenoperating before the setting may be reflected on the setting of the RFunit 32 and the signal processing unit 5. This can make the activationprocess faster.

For example, the signal processing setting unit 64 in the control unit 6may read various settings, such as the frequency band, system bandwidth,frame structure, CP length, cell ID and gain setting, from theoperational contents of the RF unit 31 and the signal processing unit 5.Alternatively, those settings may be detected by the communication statedetecting unit 62, and the signal processing setting unit 64 may beinformed of the detection result.

With the RF unit 32 enabled, the terminal 0 has a 4-antennasconfiguration and can process signals of four lines. The terminal 0receives the RS from the base station after performing predeterminedsetting, and informs the base station of the PMI, RI and CQI. The basestation informs the terminal 0 of the setting, and initiates 4×4MIMObased communication when the radio wave environment is satisfactory. Aportable terminal device which can achieve 4×4 MIMO can be provided thisway, thus ensuring communication with the maximum throughput in the DLof LTE. Of course, enabling the RF unit 32 and increasing paths(channels) increase the load on the signal processing unit 5, increasingdissipation power. To cope with it, the RF unit 32 may be disabledproperly.

FIG. 6 is a flowchart illustrating an example of a process of changingthe antenna configuration. In the process illustrated in FIG. 6, thecontrol unit 6 of the terminal 0 checks the amount of data to becommunicated and the remaining amount of the battery when setting isstarted (S1001). At this time, it is determined whether the amount ofdata to be communicated and the remaining amount of the battery arelarge or small (S1002). For example, reference values for determiningwhether the amount of data to be communicated and the remaining amountof the battery are large or small are prestored in the control unit 6,and the communication state detecting unit 62 determines whether theamount of data to be communicated and the remaining amount of thebattery are equal to or less than the reference values. When the amountof data to be communicated and the remaining amount of the battery areequal to or less than the reference values, it is determined that theamount of data to be communicated and the remaining amount of thebattery are small.

When it is determined that the amount of data to be communicated and theremaining amount of the battery are small (S1002; Yes), the control unit6 changes over the antenna selection unit 2, the RF switch 4, etc. toset two antennas to be selected by using the using-antenna selectionsetting unit 63, the signal processing setting unit 64, etc. (S1003).Accordingly, the control unit 6 sets the antenna configuration toconnect the antenna 11 and the antenna 12 to the RF unit 31. The signalprocessing setting unit 64 in the control unit 6 disables the RF unit 32which is not connected to the selected antennas (S1005), and initiatesthe communication process (S1006).

When at least one of the amount of data to be communicated and theremaining amount of the battery is larger than the reference value,i.e., when the amount of data is large or the remaining amount of thebattery is large (S1002; No), on the other hand, the control unit 6changes over the antenna selection unit 2 and the RF switch 4 to setfour antennas to be selected (S1004). In this case, the signalprocessing setting unit 64 in the control unit 6 initiates thecommunication process without disabling the RF unit 31 and the RF unit32 (S1006).

As apparent from the above, when the amount of data to be communicatedis small or the remaining amount of the battery is small, power can besaved by disabling the RF unit 32 or reducing the load on the signalprocessing unit 5. The antenna configuration may be changed according towhether the terminal 0 is connected to an AC power supply or not. Whenthe terminal 0 is connected to an AC power supply, for example, thequantity of antennas to be selected through the process of S1004 may beset to four, whereas when the terminal 0 is not connected to an AC powersupply, the quantity of antennas to be selected through the process ofS1003 may be set to two.

When the remaining amount of the battery is small although the amount ofdata to be communicated is large, the base station or a counterpartterminal or the like may be notified that communication is not possible,so that the communication process will not be initiated. When theremaining amount of the battery is large although the amount of data tobe communicated is small, the quantity of antennas to be used andwhether the RF units are usable or not may be set based on designationmade by the user of the terminal 0.

Subsequently, a description will be given of a case where the basestation has two antennas. FIG. 5B exemplifies a case where a basestation 102 has two antennas 112. In this example, the maximum number ofspatial multiplexing is two. The E-UTRA standards do not support a casewhere the quantity of antennas in the terminal is larger than thequantity of antennas in the base station, as in the case of 2×4 MIMOwhere the quantity of antennas in the base station is two and thequantity of antennas in the terminal 0 is four. From the viewpoint ofpower saving, therefore, it is desirable that the terminal 0 shouldavoid taking a 4-line configuration provided by the antennas 31, the RFunit 32 and the signal processing unit 5 when the quantity of antennasin the base station is two. For example, the using-antenna selectionsetting unit 63 in the control unit 6 sets only two antennas to be used,namely, the antenna 11 and the antenna 12, and disables the RF unit 32by means of the signal processing setting unit 64 or the like to stopfeeding power to the RF unit 32 which is not used. In addition, thesetting to allow the signal processing unit 5 to perform 2-line signalprocessing can reduce the load on the signal processing unit 5, therebyreducing dissipation power. As apparent from the above, antennas whosequantity is equal to or less than the quantity of antennas in thecounterpart base station to be communicated should be selected in theterminal 0 under control of the control unit 6 to determine the antennaconfiguration for spatial-multiplexing based communication.

In the above example, the antenna 11 and the antenna 12 are selectedwhen the quantity of antennas to be used in the terminal 0 is two. Inthe case of the MIMO that uses a plurality of antennas, the lower thecorrelation between antennas is, the greater the effect of spatialmultiplexing can be expected. Accordingly, the description will be givenof configuration examples and operational examples of the embodiment forselecting a combination of antennas with low correlation in case ofselectively using some of the four antennas, namely, the antenna 11, theantenna 12, the external antenna 13 and the external antenna 14, whichcan be used by the terminal 0. It is generally known that as thedistance between antennas becomes longer, the correlation therebetweengets lower. Therefore, if antennas which maximizes the physicalinter-antenna distance, such as the antenna 11 and the external antenna14 shown in FIG. 2, are selected, the correlation between the antennasbecomes lower, which provides a satisfactory configuration for carryingout spatial-multiplexing based communication.

In addition to the antenna 11 and the external antenna 14, for example,one of the antenna 12 and the external antenna 13 may be selectivelyused. In this case, the distances among each of the antenna 12 and theexternal antenna 13 and the antenna 11 and the external antenna 14 whichhave already been selected should be specified, and the one whichprovides a larger sum of the specified inter-antenna distances should beselected. That is, a combination of antennas to be used should beselected in such a way that the sum of the physical inter-antennadistances becomes maximum. The selection of antennas which maximize thesum of the inter-antenna distances increases the isolation betweenantennas and reduces the coupling between antenna's elements, therebyimproving the precision of calculating the correlation coefficient thatindicates the degree of correlation. In this manner, antennas to be usedshould be selected based on the physical distances among a plurality ofantennas to determine the antenna configuration for carrying outspatial-multiplexing based communication.

It is known that the correlation between antennas whose polarizationsare orthogonal to (different from) each other is low. As another exampleof selecting a combination of antennas with low correlation,configuration examples and selection operations which take polarizationinto account will be described.

In a configuration example shown in FIG. 7A, the antenna 11 and theantenna 12 are antennas for vertical polarization, which are vertical tothe ground. On the other hand, the external antenna 13 and the externalantenna 14 are antennas for horizontal polarization, which arehorizontal to the ground. In this case, the antenna 11 and the antenna12 whose transmitted/received signals have the identical polarizationdirection constitute one antenna set (first antenna set). The externalantenna 13 and the external antenna 14 whose transmitted/receivedsignals have the identical polarization direction constitute anotherantenna set (second antenna set). The first antenna set and the secondantenna set are arranged in such a way that the polarization directionof a signal transmitted/received by the antenna 11 or the antenna 12included in the first antenna set is orthogonal to the polarizationdirection of a signal transmitted/received by the external antenna 13 orthe external antenna 14 included in the second antenna set. The secondantenna set including the external antenna 13 and the external antenna14 is configured to be attachable to and detachable from the terminal 0.In such a configuration example, the combination of the antenna 11 orthe antenna 12 and the external antenna 13 or the external antenna 14can make the correlation between two antennas lower. That is, arbitraryselection of one antenna included in the first antenna set and oneantenna included in the second antenna set provides a combination ofantennas with low correlation, so that a satisfactoryspatial-multiplexing based communication can be provided. The firstantenna set and the second antenna set may be replaced with each other.

In a configuration example shown in FIG. 7B, the antenna 11 and theexternal antenna 13 are antennas for vertical polarization, which arevertical to the ground. On the other hand, the antenna 12 and theexternal antenna 14 are antennas for horizontal polarization, which arehorizontal to the ground. In this case, the antenna 11 and the antenna12 whose polarization directions are orthogonal to each other arelocated adjacent to each other to constitute one antenna set (firstantenna set). The external antenna 13 and the external antenna 14 whosepolarization directions are orthogonal to each other are locatedadjacent to each other to constitute another antenna set (second antennaset). The second antenna set including the external antenna 13 and theexternal antenna 14 is configured to be attachable/detachable to/fromthe terminal 0. In such a configuration example, the combination of theantenna 11 or the external antenna 13 and the antenna 12 or the externalantenna 14 can reduce the correlation between two antennas. That is,proper selection of one antenna included in the first antenna set or thesecond antenna set, and one antenna included in the first antenna set orthe second antenna set provides a combination of antennas with lowcorrelation, thus providing a satisfactory spatial-multiplexing basedcommunication. In addition, even when the external antenna 13 and theexternal antenna 14 are not connected to the terminal 0, the antenna 11and the antenna 12 if selected can be a combination of antennas with lowcorrelation. The first antenna set and the second antenna set may bereplaced with each other.

FIGS. 8A to 8C show configuration examples of antennas laid outthree-dimensionally. Antennas 311 to 314 shown in FIGS. 8A to 8C arelinear polarization antennas. Arrows in the diagrams represent thedirections of polarizations corresponding to the respective antennas.

FIG. 8A shows the configuration of three antennas with an antenna 312 inthe x direction, an antenna 313 in the y direction and an antenna 311 inthe z direction laid out so as to be orthogonal (90°) to one another tomake the correlation between antennas lower. FIG. 8B shows theconfiguration of four antennas laid out three-dimensionally. When thefour antennas are arranged two-dimensionally and four antennas areselected (all antennas are selected) as shown in FIGS. 7A and 7B, thereare two pairs of antennas which are not orthogonal. In the antennaconfiguration shown in FIG. 8B, an antenna 314 in the −y direction isdisposed in addition to the antenna configuration shown in FIG. 8A.Therefore, antennas which do not orthogonal to each other are a pair ofthe antenna 313 and antenna 314 whose polarization directions are thesame, making it possible to reduce a pair of antennas whose polarizationdirections are not orthogonal to each other as compared with thetwo-dimensional layout. FIG. 8C shows a configuration example where fourantennas are laid out so as to cross one another at angles of 120°. Inthis configuration example, while the antennas are not exactlyorthogonal to one another, the polarization directions do not becomeidentical, so that the correlation between the antennas can be setlower.

The combination of some or all of the antenna 11, the antenna 12, theexternal antenna 13 and the external antenna 14 or the combination ofthe antennas 311 to 314 allows antennas with orthogonal (different)polarizations to be selectively used. This can provide the antennaconfiguration which reduces the correlation between a plurality ofantennas and is satisfactory for carrying out spatial-multiplexing basedcommunication.

For example, in case of reading setting data indicative of the physicaldistances among the individual antennas or setting data indicative ofthe directions of polarizations corresponding to the respective antennasto select some (a plurality of antennas) of all the antennas, theantenna configuration specifying unit 61 in the control unit 6 shouldselect antennas to be used according to the physical configurations ofthe individual antennas in such a way as to reduce the correlation amonga plurality of antennas.

The method of specifying the physical configurations of a plurality ofantennas is not limited to reading setting data prepared in advance, anda predetermined measurement may be performed. FIG. 9 is a flowchartillustrating an example of a process of selecting antennas to be usedaccording to the physical configurations of individual antennasspecified based on the result of the predetermined measurement. In thisexample, the antenna 11 is preset (fixed) as an antenna to be used.

When the process shown in FIG. 9 starts, the control unit 6, e.g., thecommunication state detecting unit 62, in the terminal 0 determineswhether it is a standby state or not, i.e., whether it is in standbymode, before measurement of RS, before initiation of communication, orno communication state, by detecting the operational state of the signalprocessing unit 5 or the like (S2001). When having determined that it isnot in standby mode (S2001; No), the control unit 6 repeatedly executesthe process of S2001. When having determined that it is in standby mode(S2001; Yes), on the other hand, the control unit 6 transmits areference signal for measurement from the antenna 11 by causing, forexample, the antenna configuration specifying unit 61 or thecommunication state detecting unit 62 to output a predetermined commandsignal to the signal processing unit 5 and the RF unit 31 (S2002).

The reference signal transmitted from the antenna 11 in S2002 isreceived by another antenna 12, external antenna 13, or external antenna14. At this time, the physical configuration of each antenna, such asthe positional relation and the degree of correlation, from thereception level, the phase, the measured reception quality, thecorrelation coefficient, etc. (some or all of them) (S2003). Then, thecontrol unit 6 selects an antenna to be used with the antenna 11 fromthe antenna 12, the external antenna 13 and the external antenna 14based on the result of measurement in S2003 (S2004 to S2006). When suchan antenna is selected based on the result of measuring the receptionlevel, for example, an antenna whose reception level for the referencesignal is the lowest is selected from the antenna 12, the externalantenna 13 and the external antenna 14. This is the antenna farthestfrom the antenna 11 that has transmitted the reference signal, or theantenna whose polarization direction provides the largest differencefrom the polarization direction of a radio signal to betransmitted/received from/at the antenna 11, and it can be assumed thatthe antenna has low correlation to the antenna 11. After antennaselection is finished this way, the control unit 6, e.g., the signalprocessing setting unit 64, performs operational setting of the RFswitch 4, the signal processing unit 5, the RF unit 31 and the RF unit32 according to the selected antenna. As a result, signal processing forcarrying out communication with the combination of the selected antennasis started (S2007).

Selecting a combination of antennas with low correlation according tothe physical configuration of each antenna specified based on the resultof such a predetermined measurement can provide a satisfactory antennaconfiguration for spatial-multiplexing based communication. When one ofthe antenna 12, the external antenna 13 and the external antenna 14 ispreset (fixed) as an antenna to be used, the reference signal formeasurement is transmitted from the antenna to be used in S2002. Theantenna with low correlation should be selected based on the result ofthe measurement performed then in S2003. The phase difference when thereference signal transmitted from the antenna 11 is received at otherantennas (antenna 12, external antenna 13 and external antenna 14) maybe measured, and the antenna with the largest phase difference may beselected. In this manner, a combination of antennas with low correlationcan be selected based on the result of a predetermined measurementperformed when the reference signal transmitted from one of the antenna11 and antenna 12 incorporated in the terminal 0, and the externalantenna 13 and external antenna 14 connected to the terminal 0 isreceived at the other antennas.

Further, a description will be given of a case where a reception qualityreport showing CQI, RI, etc., for example, as another example ofselecting a combination of antennas to be used in spatial-multiplexingbased communication. FIG. 10 is a flowchart illustrating an example of aprocess of selecting antennas to be used based on a predeterminedreception quality report. It is assumed in this example too that theantenna 11 is preset (fixed) as an antenna to be used as in the processexample illustrated in FIG. 9.

In the process illustrated in FIG. 10, first, the antenna 12 is selected(S3001). Then, a reception quality report when the antenna 11 and theantenna 12 are used is held (S3002). For example, the communicationstate detecting unit 62 in the control unit 6 should predict the DLchannel characteristic from the result or the like of executing signalprocessing in the signal processing unit 5, and prepare a receptionquality report showing the predicted DL channel characteristic. Afterthe reception quality report is held in S3002, selection of the antenna12 is stopped, and the external antenna 13 is selected (S3003).Subsequently, a reception quality report when the antenna 11 and theexternal antenna 13 are used is held (S3004). After the receptionquality report is held in S3004, selection of the external antenna 13 isstopped, and the external antenna 14 is selected (S3005). At this time,a reception quality report when the antenna 11 and the external antenna14 are used is held (S3006). In this manner, all reception qualitiescorresponding to cases where any one of the antenna 12, the externalantenna 13 and the external antenna 14 is used with the antenna 11 aredetected, and a reception quality report showing those detection resultsis held. Thereafter, the reception quality reports are compared with oneanother. Then, an antenna with the best reception quality is determinedin the antenna combinations with the antenna 11 based on the result ofthe comparison (S3007). Thereafter, an antenna to be used with theantenna 11 is selected (S3008 to S3010). After antenna selection isfinished this way, the signal processing setting unit 64, for example,performs operational setting of the RF switch 4, the signal processingunit 5, the RF unit 31 and the RF unit 32 according to the selectedantenna. As a result, signal processing for carrying out communicationwith the combination of the selected antennas is started (S3011).

Selecting a combination of antennas with low correlation based on theresult of comparison of such reception quality reports can provide asatisfactory antenna configuration for spatial-multiplexing basedcommunication. The reception quality report should show arbitrarymeasurements which can specify the DL channel characteristic, and mayshow, for example, signal to interference and noise ratio (SINR), thebit error ratio (BER), the energy per bit to noise power spectraldensity ratio (Eb/No) or the like.

Next, configuration examples and operational examples when a portableterminal device includes a sensor will be described. In theconfiguration examples shown in FIGS. 1A and 1B, the terminal 0 includesa sensor 8. The sensor 8 is, for example, an azimuth sensor using anacceleration sensor or the like, which can detect the inclination of theterminal 0. The sensor 8 may be provided as separate from the controlunit 6 in the terminal 0, or may be provided inside the control unit 6as part of the communication state detecting unit 62, for example.Further, the sensor 8 is not limited to the type which is incorporatedin the terminal 0, and may be externally connected to the terminal 0 totransfer data indicative of the detection result to the control unit 6.

In this configuration example, the terminal 0 incorporates an antenna 15and an antenna 16 in addition to the antenna 11 and the antenna 12. Theantenna 15 and the antenna 16 are not limited to the type which isincorporated in the terminal 0, and, like the external antennas 13 and14, may be configured as detachable external antennas. In addition,terminals 0 shown in FIGS. 11A and 11B each include a processing unit 50which has all the functions of the foregoing RF units 31 and 32 and thesignal processing unit 5. That is, the processing unit 50 can processsignals of four lines (four high-frequency signals).

When the terminal 0 is placed in the direction as shown in FIG. 11A, theantenna 11 and the antenna 12 become antennas for vertical polarization,and the antenna 15 and the antenna 16 become antennas for horizontalpolarization. As one example of the operation in this case, thecombination of the antennas 11 and 12 provides an antenna configurationwhich maximizes the inter-antenna distance, has low correlation and issatisfactory for receiving vertical polarized signals.

When the terminal 0 is tilted and placed in the direction as shown inFIG. 11B thereafter, the antenna 15 and the antenna 16 which have beenhorizontal become vertical to be able to receive vertical polarizedsignals. At the time the use of the terminal 0 views a streaming video,for example, the layout of the terminal 0 may be changed to change thescreen direction from the vertical screen for selecting a program to thehorizontal screen for viewing a video. At this time, the inclination ofthe terminal 0 is detected by the sensor 8, and when a certain orgreater inclination is detected, the control unit 6 changes the antennasto be used. As one example, upon detection of a change of 45° or greaterfrom the initial value (inclination angle) when the antennaconfiguration has been determined first, the antennas to be used shouldbe changed to the antennas 15 and 16 from the antennas 11 and 12.Alternatively, the combination of antennas to be used may be changedaccording to whether the state of the terminal 0 in use is the verticalscreen or the horizontal screen.

As apparent from the above, the antenna configuration can be changed sothat the inclination of the terminal 0 is detected by the sensor 8 toalways ensure reception of. The antenna configuration can be changedflexibly based on the result of detecting that the layout of theterminal 0 (position and direction, the state or the like) has beenchanged, thereby achieving satisfactory spatial-multiplexing basedcommunication. The polarization direction of signals to betransmitted/received normally is not limited to the vertical direction,and may be a predetermined arbitrary direction such as the horizontaldirection.

In the above example, the initial setting of the antenna configurationwhich has low correlation and can carry out satisfactory communicationis set to the state where the antennas 11 and 12 for verticalpolarization are selected for signal reception based on theinter-antenna distance. Alternatively, a combination of antennas whichprovides a satisfactory antenna configuration may be predicted and setso that, for example, the initial setting state is set to the statewhere the antenna 11 for vertical polarization and the antenna 15 forhorizontal polarization are selected for signal reception based on thepolarization directions of signals to be transmitted/received. Settingthe combination of antennas to the initial setting state can shorten theset time at the time of activation or can ensure previous setting inconsideration of the characteristic unique to the device.

The sensor 8 is not limited to the type which detects the inclination ofthe terminal 0, and may be of a type which can detect an arbitraryamount of variation in the communication environment of the terminal 0.As one specific example, a speed sensor for a vehicle, a range sensortherefor, or the like may be configured as an external sensor 8 and dataindicative of the detection result may be transferred to the controlunit 6 when the terminal 0 is mounted to the vehicle. The control unit 6may execute a process of determining whether a predetermined amount ofvariation in the communication environment of the terminal 0 is equal toor less than a predetermined reference value, or is greater than thereference value. In addition, the setting of the cycle of changing thecombination of antennas to be used may be varied according to whetherthe predetermined amount of variation in the communication environmentof the terminal 0 has exceeded the reference value or not. For example,the control unit 6 should vary the setting of the cycle of executing theprocess as shown in, for example, FIG. 6, FIG. 9 or FIG. 10 according towhether the predetermined amount of variation in the communicationenvironment of the terminal 0 has exceeded the reference value or not.That is, regardless of the case where the combination of antennas to beused is changed based on the physical configurations of the individualantennas, or the case where the combination of antennas to be used ischanged based on the result of detecting the reception quality, thechanging cycle should be varied according to a predetermined amount ofvariation in communication environment.

The following operation may take place as one example of such anoperation of varying the cycle of changing the antenna configurationbased on a predetermined amount of variation in communicationenvironment. When the amount of variation detected is equal to or lessthan the reference value and there is a little change in communicationenvironment, for example, the cycle of changing the antennaconfiguration is set to a predetermined first cycle. When the amount ofvariation detected exceeds the reference value to show a large change incommunication environment, on the other hand, the cycle of changing theantenna configuration is set to a second cycle shorten than the firstcycle. Accordingly, when there is a little change in the communicationenvironment of the terminal 0 (e.g., when the moving speed of theterminal 0 is slow), the frequency of executing the process of changingthe antenna configuration can be reduced to prevent the dissipationpower from increasing. When there is a large change in the communicationenvironment of the terminal 0 (e.g., when the moving speed of theterminal 0 is fast), the frequency of executing the process of changingthe antenna configuration can be increased to quickly cope with thevarying communication environment and change the antenna configuration,so that a satisfactory communication state can be maintained.

As one example of the operation of varying the cycle of changing theantenna configuration based on a predetermined amount of variation incommunication environment, the amount of variation which has beendetected regularly, for example, may be recorded, and the setting of thecycle of changing the antenna configuration may be varied according tothe characteristic of the time-dependent change. When a time-dependentchange in the amount of variation detected is equal to or less than thereference value and the communication environment is changing bysubstantially a constant change, for example, the cycle of changing theantenna configuration is set to a predetermined third cycle. When thetime-dependent change in the amount of variation detected exceeds thereference value so that the communication environment is changing by arandom change, on the other hand, the cycle of changing the antennaconfiguration is set to a fourth cycle longer than the third cycle. Thatis, when the time-dependent change in the amount of variation in thecommunication environment of the terminal 0 is large and the amount ofvariation in the moving distance, the moving direction or the like ischanging at random, the frequency of executing the process of changingthe antenna configuration is reduced. This can prevent frequent changesbetween specific (e.g., two or three) antenna configurations, such asthe antenna configuration returning to the original antennaconfiguration after being changed to another antenna configuration, thuspreventing an increase in dissipation power. When there is a littlechange in the amount of variation in the communication environment ofthe terminal 0 and the amount of variation in the moving distance, themoving direction or the like is changing substantially constantly, thefrequency of executing the process of changing the antenna configurationis increased. This makes it possible to quickly cope with the varyingcommunication environment and change the antenna configuration, therebymaintaining a satisfactory communication state.

According to the invention, as described above, when antennas whosequantity is equal to or less than the quantity of antennas of a basestation to be communicated are selected to carry outspatial-multiplexing based communication, the setting is made to selecta combination of antennas with low correlation, and stop power supply tothe RF units which corresponds to the antennas which are not selected,thereby reducing the load on the signal processing unit. This makes itpossible to change the antenna configuration to the one which canprovide a satisfactory spatial-multiplexing based communication statewhile reducing the dissipation power, thereby improving the convenience(user-friendliness) and stability of the radio communication apparatus.

To select antennas with low correlation, a combination of antennas to beused should be selected based on the physical configurations of theindividual antennas, as in the case of selecting a combination ofantennas whose inter-antenna distance becomes maximum or the case ofselecting a combination of antennas which transmit/receive radio signalswhose polarization directions differ from one another. This can permitthe antenna configuration to be changed flexibly and provide asatisfactory spatial-multiplexing based communication state. To specifya combination of antennas with low correlation according to the physicalconfigurations of the individual antennas, the reference signal formeasurement which has been transmitted from one antenna may be receivedat other antennas, and the level of receiving the reference signal, thephase thereof, the reception quality and the like may be measured.

A combination of antennas to be used may be changed based on theoperational state of the radio communication apparatus, such as whetherthe amount of data to be communicated is large or small, or theremaining amount of the battery is large or small. This makes itpossible to reduce the quantity of antennas to be used and the loads onthe RF unit and the signal processing unit, thereby preventing thedissipation power from increasing, when the amount of data to becommunicated is small, or when the remaining amount of the battery issmall.

The invention is not limited to the foregoing embodiment, and variousmodifications and applications can be made thereto. For example, theforegoing description of the embodiment has been given of the case theRF unit 31 is enabled while the RF unit 32 is disabled when the quantityof antennas in a base station is two. However, the invention is notlimited to this case, and even when the quantity of antennas in a basestation is two, both of the RF units 31 and 32 may be enabled so thatthe RF unit 32 and the signal processing unit 5 ensure simultaneous usewith another network system (e.g., UMTS, CDMA, HRPD, EvDO, GPS, orBluetooth), monitoring, handover, or the like. To realize such anoperation, the RF unit 31, the RF unit 32 and the signal processing unit5 should be configured by a multi-mode IC capable of coping with pluraltypes of network systems.

The following will describe connection to another network system. Inthis example, the terminal 0 uses the antenna 11, and selectively usesone of the antenna 12, the external antenna 13 and the external antenna14. It is assumed that the combination of the antenna 11 and the antenna12 is selected and data communication is carried out using the RF unit31 and the signal processing unit 5. At this time, signal processing forexecuting communication in another network system is performed by the RFunit 32 or the like using the external antenna 13 and the externalantenna 14 which are not unused.

As a specific example, the RF unit 32 may execute signal processing formaking a voice call using the CDMA. This can permit simultaneousexecution of data communication using the LTE and a voice call using theCDMA, thus improving the convenience of the terminal 0. The RF unit 32has two processing circuits which are capable of frequency-convertingtwo high-frequency signals. When communication is feasible in anothernetwork system different from the LTE using only one processing circuit,an antenna (e.g., external antenna 14) whose inter-antenna distancebecomes physically maximum as viewed from the antenna that is using theLTE should be selected as an antenna to be used. Apparently, even whencommunication is carried out in a plurality of network systems, antennaswith low correlation among the individual network systems are selectedto minimize the influence on the communication in the network systems,thereby ensuring satisfactory communication.

Other network systems which are used with the LTE may be given prioritylevels, so that a network system to be used can be set properlyaccording to various situations. Given that network systems to be usedare set with different priority levels according to the time, place,moving speed, etc. beforehand, for example, it is possible to shortenthe connection delay and stabilize communication. When using anothernetwork system significantly degrades the existing communicationperformance provided by the LTE, the procedure to connect to anothernetwork system may be interrupted.

When LTE-based communication is carried out with the combination of theantennas 11 and 12 selected, the external antenna 13 and the externalantenna 14 which are not used, and the RF unit 32 may be used to acquirecell information needed to select a cell again. When the current cellregistered in the terminal 0 is not optimal, it is necessary to select acell again. In this case, various measurements and decisions on thestates of peripheral cells are made so that a certain interruption time(delay time) is generated by re-establishment of a link, random accessand the like. According to the LTE, when reselection of a cell isperformed without the current cell's fulfilling a predetermined decisioncriterion, a maximum time of 100 ms is needed.

To cope with it, cell information needed for re-selection of a cell isacquired prior to the re-selection process, and the priority levels ofcells are determined so as to shorten the time (interruption time)needed to resume communication through re-establishment of a link or thelike. As one example, the priority levels of cells should be applied tore-selection of a cell using the RF unit 31 or the like while LTE-basedcommunication is taking place. As another example, the communicationoperation may be changed over in such a way that communication isstarted using the RF unit 32 or the like according to the prioritylevels of cells based on the cell information acquired using the RF unit32 or the like, and the operation of the RF unit 31 is stopped(disabled).

The external antenna 13 and the external antenna 14 according to theforegoing embodiment need not be configured as dedicated externalantennas, but may be mounted to, for example, a cradle 9 as shown inFIG. 12. The connection of the external antenna 13 or the externalantenna 14 with the terminal 0 may be established when the terminal 0 isheld into the cradle 9 which may be is used to charge and set theterminal 0 and perform communication therewith, e.g., when a connector91 is fitted into the connector unit 7.

According to the embodiment, the terminal 0 has a 4-line configurationto be able to use four antennas at a maximum and have two 2-line RFunits (RF unit 31 and RF unit 32). However, the invention is not limitedto this configuration, and the maximum quantity of antennas and thenumber of lines of the transmission/reception circuits, the quantitythereof, and so forth can be optionally set according to thespecifications for executing spatial-multiplexing based communication.For example, the terminal 0 may have an 8-line configuration includingeight 1-line RF units, or an 8-line configuration including a single8-line RF unit.

The invention can be worked out using a computer that controls not onlya dedicated radio communication apparatus, but also an ordinary radiocommunication apparatus which performs spatial-multiplexing basedcommunication. That is, a program for allowing the computer thatcontrols a radio communication apparatus to function as the foregoingcomponents to execute the foregoing processes may be recorded on apredetermined recording medium so that a microprocessor such as a CPUreads out and executes the program to allow the computer to function asthe radio communication apparatus according to the embodiment. Such aprogram may be recorded in a computer readable recording medium, such asa FD, CD, DVD, MO or IC memory, for distribution. Further, the programmay be stored in a file system included in an FTP (File TransferProtocol) server or the like provided on an electric communicationnetwork such as the Internet, and may be downloaded into a computer inthe form of a signal superimposed on a carrier wave.

Having described and illustrated the principles of this application byreference to one or more preferred embodiments, it should be apparentthat the preferred embodiment(s) may be modified in arrangement anddetail without departing from the principles disclosed herein and thatit is intended that the application be construed as including all suchmodifications and variations insofar as they come within the spirit andscope of the subject matter disclosed herein.

What is claimed is:
 1. A radio communication apparatus including anantenna set including a plurality of antennas and executingspatial-multiplexing based communication, comprising: an antennaselection unit that selects from the antennas a combination of antennaswhose quantity is equal to or less than a quantity of counterpartantennas to be communicated, as a combination of antennas to be used inspatial-multiplexing based communication; a signal processing unit thatexecutes signal processing for communication using the combination ofantennas selected by the antenna selection unit; a reception qualitydetecting unit that detects a reception quality for each combination ofa plurality of antennas; and a communication environment changedetecting unit that detects a time-dependent change in a predeterminedamount of variation in communication environment of the radiocommunication apparatus, wherein the antenna selection unit selects thecombination of antennas to be used according to a result of comparisonof reception qualities detected by the reception quality detecting unit,and varies a period of changing a combination of antennas to beselected, depending on whether the time-dependent change in the amountof variation detected by the communication environment change detectingunit is equal to or less than a predetermined reference value.
 2. Theradio communication apparatus according to claim 1, wherein the signalprocessing unit includes a plurality of signal processing circuits thatprocess signals to be transmitted/received in association with theplurality of antennas included in the antenna set, and power supply toone of the plurality of signal processing circuits which is not used isstopped based on the combination of antennas selected by the antennaselection unit.
 3. The radio communication apparatus according to claim1, wherein the signal processing unit includes a plurality of signalprocessing circuits that process signals to be transmitted/received inassociation with the plurality of antennas included in the antenna set,and a setting value for carrying out communication with the antennas tobe communicated or another antenna to be communicated is acquired byusing an antenna which is not selected by the antenna selection unit anda signal processing circuit which is associated with the antenna.
 4. Theradio communication apparatus according to claim 1, wherein the antennaset includes: a first antenna set including a plurality of firstantennas in which polarization directions of adjacent antennas areorthogonal to each other; and a second antenna set including a pluralityof second antennas in which polarization directions of adjacent antennasare orthogonal to each other.
 5. The radio communication apparatusaccording to claim 4, wherein the first antenna set or the secondantenna set is configured to be dismountable from the radiocommunication apparatus, and the radio communication apparatus furthercomprises a mount detection unit that detects if the first antenna setor the second antenna set is mounted to the radio communicationapparatus.
 6. The radio communication apparatus according to claim 1,wherein the antenna set includes: a first antenna set including aplurality of first antennas, which make polarization directions ofsignals to be transmitted/received identical to each other; and a secondantenna set including a plurality of second antennas, which makepolarization directions of signals to be transmitted/received identicalto each other, and the polarization direction of the signals to betransmitted/received by the plurality of first antennas is orthogonal tothe polarization direction of the signals to be transmitted/received bythe plurality of second antennas.
 7. The radio communication apparatusaccording to claim 6, wherein the first antenna set or the secondantenna set is configured to be dismountable from the radiocommunication apparatus, and wherein the radio communication apparatusfurther comprises a mount detection unit that detects if the firstantenna set or the second antenna set is mounted to the radiocommunication apparatus.
 8. The radio communication apparatus accordingto claim 1, wherein the antenna selection unit selects a combination ofantennas in such a way as to always transmit/receive a signal having acertain polarization direction.
 9. A radio communication apparatusincluding an antenna set including a plurality of antennas and executingspatial-multiplexing based communication, comprising: antenna selectionmeans that selects from the antennas a combination of antennas whosequantity is equal to or less than a quantity of counterpart antennas tobe communicated, as a combination of antennas to be used inspatial-multiplexing based communication; signal processing means thatexecutes signal processing for communication using the combination ofantennas selected by the antenna selection means; reception qualitydetecting means that detects a reception quality for each combination ofa plurality of antennas; and communication environment change detectingmeans that detects a time-dependent change in a predetermined amount ofvariation in communication environment of the radio communicationapparatus, wherein the antenna selection means selects the combinationof antennas to be used according to a result of comparison of receptionqualities detected by the reception quality detecting means, and variesa period of changing a combination of antennas to be selected, dependingon whether the time-dependent change in the amount of variation detectedby the communication environment change detecting means is equal to orless than a predetermined reference value.
 10. A radio communicationmethod of executing spatial-multiplexing based communication using anantenna set including a plurality of antennas, comprising: an antennaselection step of selecting from the antennas a combination of antennaswhose quantity is equal to or less than a quantity of counterpartantennas to be communicated, as a combination of antennas to be used inspatial-multiplexing based communication; a signal processing step ofexecuting signal processing for communication using the combination ofantennas selected in the antenna selection step; a reception qualitydetecting step of detecting a reception quality for each combination ofa plurality of antennas; and a communication environment changedetecting step that detects a time-dependent change in a predeterminedamount of variation in communication environment of the radiocommunication apparatus, the antenna selection step selecting thecombination of antennas to be used according to a result of comparisonof reception qualities detected in the reception quality detecting step,and varying a period of changing a combination of antennas to beselected, depending on whether the time-dependent change in the amountof variation detected in the communication environment change detectingstep is equal to or less than a predetermined reference value.
 11. Anon-transitory computer readable medium storing a program allowing acomputer that controls a radio communication apparatus which executesspatial-multiplexing based communication using an antenna set includinga plurality of antennas, to function as: a reception quality detectingunit that detects a reception quality for each combination of aplurality of antennas; an antenna selection unit that selects from theantennas a combination of antennas whose quantity is equal to or lessthan a quantity of counterpart antennas to be communicated, as acombination of antennas to be used in spatial-multiplexing basedcommunication, according to a result of comparison of receptionqualities detected by the reception quality detecting unit; a signalprocessing unit that executes signal processing for communication usingthe combination of antennas selected by the antenna selection unit; anda communication environment change detecting unit that detects atime-dependent change in a predetermined amount of variation incommunication environment of the radio communication apparatus, whereinthe antenna selection unit varies a period of changing a combination ofantennas to be selected, depending on whether the time-dependent changein the amount of variation detected by the communication environmentchange detecting unit is equal to or less than a predetermined referencevalue.